Really? #header is quite common and a plugin shouldn't use it.
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RufusJan 25 '12 at 16:20

thats what I thought too luckily I speak spanish so I just put "cabecera" lol
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AndresJan 25 '12 at 16:31

Sorry but disagree. Fancybox doesn't add any wrapper with the ID #header. All fancybox wrappers use the prefix "fancybox" like #fancybox-outer, #fancybox-iframe, etc. This looks more like a css issue than a fancybox issue.
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JFKJan 25 '12 at 17:45

@JFK If that solved the issue what can it be? How do you know what "fancybox" script used in the code (I'm aware of the one that's really the Fancybox, but the OP could use 'fancybox' just as a common name for that kind of scripts). It might be even a custom written by his buddy, that actually adds #header :)
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Zoltan TothJan 25 '12 at 17:58

@ZoltanToth this is definitively a CSS conflict issue but not created by fancybox, which doesn't append any #header to the DOM. If Andres shares a link with a sample of the issue, I could tell you exactly what the problem is.
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JFKJan 25 '12 at 18:19